avcodec/svq1dec: Check that there is data left after the header

Fixes: Timeout (21sec -> 255ms)
Fixes: 20709/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SVQ1_fuzzer-5085075089915904

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/svq1dec.c

@@ -602,6 +602,8 @@ static int svq1_decode_frame_header(AVCodecContext *avctx, AVFrame *frame)
         if (skip_1stop_8data_bits(bitbuf) < 0)
             return AVERROR_INVALIDDATA;
     }
+    if (get_bits_left(bitbuf) <= 0)
+        return AVERROR_INVALIDDATA;
 
     s->width  = width;
     s->height = height;